Power amplifiers are widely used in communication systems, for example in radio base stations and cellular phones of a cellular radio network. In such cellular radio network, power amplifiers typically amplify signals of high frequencies for providing a radio transmission signal. A consideration in the design of power amplifiers is the efficiency thereof. High efficiency is generally desirable so as to reduce the amount of power that is dissipated as heat. Moreover, in many applications, such as in a satellite or a cellular phone, the amount of power that is available may be limited due to powering by a battery, included in e.g. the satellite. An increase in efficiency of the power amplifier would allow an increase of operational time between charging of the battery.
A conventional Power Amplifier (PA), such as class B, AB, F, has a fixed Radio Frequency (RF) load resistance and a fixed voltage supply. Class B or AB bias causes the output current to have a form close to that of a pulse train of half wave rectified sinusoid current pulses. The Direct Current (DC), and hence DC power, is largely proportional to the RF output current amplitude, and voltage. The output power, however, is proportional to the RF output current squared. An efficiency of the conventional power amplifier, i.e. output power divided by DC power, is therefore also proportional to the output amplitude. The average efficiency is consequentially low when amplifying signals that on average have a low output amplitude, or power, compared to the maximum required output amplitude.
Known RF power amplifiers include both Doherty and Chireix type power amplifiers. These kinds of RF PAs are generally more efficient than the conventional amplifier described above for amplitude-modulated signals with high Peak-to-Average Ratio (PAR), since they have a lower average sum of output currents from the transistors. Reduced average output current means high average efficiency.
The reduced average output current is obtained by using two transistors that influence each other's output voltages and currents through a reactive output network, which is coupled to a load. By driving the constituent transistors with the right amplitudes and phases, the sum of RF output currents is reduced at all levels except the maximum. Also for these amplifiers the RF voltage at one or both transistor outputs is increased.
Generally, RF power amplifier can be driven in a so called backed off operation. This means that the power amplifier is operated a certain level, e.g. expressed as a number of decibels (dBs), under its maximum output power. Backed off operation may also refer to that an instantaneous output power is relatively low.
Referring to FIG. 1, WO03/06111 discloses a composite power amplifier 10 including a first and a second power amplifier 11, 12 connected to an input signal over an input network and to a load RLOAD over an output network 13. The output network 13 includes a longer and a shorter transmission line 14, 15 for generating different phase shifts from each power amplifier output to the load RLOAD. Each of the longer and shorter transmission lines 14, 15 connects each of the first and second amplifiers 11, 12 to a common output at the load RLOAD. In order to achieve, for this composite power amplifier 10, a widest wideband operation, lengths of the longer and shorter transmission lines 14, 15 are chosen such that the longer transmission line 14 has an electrical length of half a wavelength at a center frequency of the composite amplifier 10, while the shorter transmission line 15 is a quarter wavelength long at the center frequency. The composite power amplifier may be operated, typically over a 3 to 1 bandwidth, in Doherty mode, in Chireix mode or in other intermediate modes between the Doherty and Chireix modes. Thus, the 3 to 1 bandwidth of high efficiency is achieved by devising an output network 13 that has both suitable impedance transformation characteristics and full power output capacity over the bandwidth. A continuous band of high efficiency amplification is thus achieved.
In FIG. 2, a simplified structure of the composite amplifier of FIG. 1 is shown. The shorter and longer transmission lines are shown as branches 21, 22 and the first and second amplifiers 11, 12 are connected to a respective branch 21, 22. The branches 21, 22 are connected to the load RLOAD.
A drawback of the above mentioned composite power amplifier is that the efficiency for signals with high PAR, e.g. 10 dB, may for some applications not be sufficient.
Moreover, the above mentioned composite power amplifier may not always achieve high efficiency over a sufficiently wide bandwidth.